The objective of the proposal is to study various physiological aspects of the life cycle of Acanthamoeba and Entamoeba with special reference to the dormant, cyst stage. Acanthamoeba is found in a variety of environments ranging from soil to mammalian tissues. Thus the various strains serve to illustrate the changes required for a free-living amoeba to become a parasite. As such they may indicate how the anaerobic, intestinal amoeba, Entamoeba evolved from an unknown, free-living form and they may provide a model applicable to elucidating the biology of Entamoeba. For convenience, the research on encystation is divided into: 1) environmental stimuli for encystation and cyst germination; 2) exogenous requirements for these; 3) biochemical changes accompanying the event; 4) mechanisms of dormancy, including resistance to deleterious, exogenous factors (e.g. drying, starvation, etc.). Since work on Acanthamoeba is further along than for Entamoeba, observations on the former will be used as a guide to the work on the latter. In addition, the biology of Entamoeba, cultured free of other organisms (e.g. bacteria, etc.), will be elucidated further as a necessary requisite for the study of the cyst. BIBLIOGRAPHIC REFERENCE: Band, R. N. and Balamuth, W. 1974. Hemin replaces serum as a growth requirement for Naegleria. Appl. Microbiol. 28, 64-5.